Yoneho Tabata

Radiation induced crosslinking of polytetrafluoroethylene

Abstract The irradiation effect of polytetrafluoroethylene (PTFE) was examined by tensile test and thermal analysis in a wide range of irradiation temperature. The tensile strength and elongation at break changed very much by irradiation temperature, especially around the PTFE melting temperature (327°C). By electron beam irradiation under vacuum in the molten state at around 340°C, the dose required for the half decay of the elongation at break was about 1 MGy, and the modulus and yield strength increased with dose, whereas the dose for the half decay at room temperature irradiation was less than 50 kGy, and the modulus and yield strength decreased. The crystallinity decreased by the irradiation in the molten state, but increased with dose below the melting temperature. It is concluded that network is formed with the crosslinking of PTFE by irradiation under vacuum in the molten state just above the melting point.

Improvement of radiation resistance for polytetrafluoroethylene (PTFE) by radiation crosslinking

Abstract Crosslinked polytetrafluoroethylene(PTFE) was prepared by electron beam irradiation in the molten state of PTFE at 340 ± 3°C in inert gas atmosphere. The crosslinking density was changed by the irradiation dose. Radiation resistance of the crosslinked PTFE was examined on the mechanical properties after γ-ray irradiation at room temperature under vacuum and in air. Dose at a half value of elongation at break was about 1 MGy for 500 kGy-crosslinked PTFE, while dose for non-crosslinked PTFE was only 3.5 kGy. It has been found that the radiation resistance of PTFE can be improved to large extent by crosslinking.

Evidence for radiation induced crosslinking in polytetrafluoroethylene by means of high-resolution solid-state 19F high-speed MAS NMR

Radiation effects on molecular structure of polytetrafluoroethylene (PTFE) were studied by high-resolution solid-state 19F high speed magic angle spinning (HS MAS) NMR spectroscopy. Samples used for the NMR studies were prepared by electron beam irradiation of PTFE with a wide range of irradiation doses from 0.5–10 MGy in the molten state at 340°C under oxygen-free atmosphere. While the non-irradiated PTFE displayed only an intense peak of the internal CF2, several new signals corresponding to CF3, CF2 and CF groups were observed for the PTFE which was high temperature irradiated at 340°C in oxygen-free atmosphere (hti-PTFE). Intensities of these new signals increased with an increase of irradiation dose. The present solid-state 19F HS MAS NMR studies provide not only the first experimental evidence regarding the existence of crosslinking structure in hti-PTFE, directly detected as the CF signal, but also the crosslinking density which can be estimated from a proportion of the CF versus total fluorine signal intensities. The higher the irradiation dose, the higher the crosslinking density; hti-PTFE with 10 MGy contains one crosslinking site per approximately 24 CF2 groups, while the hti-PTFE with 5 MGy contains one crosslinking site per approximately 36 CF2 groups. Further, G value of crosslinking (G(x)) was estimated from the signal intensities of 19F HS MAS NMR spectra. The highest G(x)-value, 1.85, was observed for the 2MGy hti-PTFE sample, suggesting that crosslinking of PTFE is formed most efficaciously with 2 MGy irradiation in the molten state at 340°C under oxygen-free atmosphere.

ESR study on free radicals trapped in crosslinked polytetrafluoroethylene (PTFE)

Abstract Free radicals in crosslinked PTFE which formed by 60 Co γ-rays irradiaion at 77 K and at room temperature were studied by electron spin resonance (ESR) spectroscopy. The crosslinked PTFE specimens with different crosslinking density were prepared by electron beam irradiation in the molten state. The ESR spectra observed in the irradiated crosslinked PTFE are much different from those in non-crosslinked PTFE (virgin); a broad singlet component increases with increasing the crosslinking density, G -value of radicals is much higher in crosslinked PTFE than in non-crosslinked one. Free radicals related to the broad component are trapped in the non-crystalline region of crosslinked PTFE and rather stable at room temperature, whereas radicals trapped in amorphous non-crosslinked PTFE are unstable at room temperature. It is thought that most of free radicals trapped in the crosslinked PTFE are formed in the crosslinked amorphous region. The trapped radicals decays around 383 K (110°C) due to the molecular motion of α-relaxation.

Temperature effects on radiation induced phenomena in polytetrafluoroetylene (PTFE) : Change of G-value

Abstract Temperature dependencies on the radiation induced phenomena and G-value of polytetrafluoroethylene (PTFE) have been studied in a temperature range 77–653 K. It is well known that main chain scission occurs very effectively below the melting temperature of PTFE (600 K). We have found in our experiments that G-value of chain scission increases significantly with increasing irradiation temperature, until 600 K. In addition to that, we have realized that crosslinking occurs by irradiation in the molten state at 613 K (Tabata, 1992; Oshima et al. , 1995; Tabata et al. , 1996). In the molten state, G-value of crosslinking was found to be 0.35 (number of crosslinking/100 eV absorption), as a lower limit, and the apparent G-value of chain scission is drastically reduced. Above 633 K, radiation induced crosslinking mainly occurs, however parallel thermal depolymerization or decomposition takes place to some extent.

Change of molecular motion of polytetrafluoroethylene (PTFE) by radiation induced crosslinking

Abstract The mechanical relaxation of radiation crosslinked PTFE was investigated in conjunction with molecular motion by means of dynamic visco-elastic property measurement in a temperature range from −150 to 350°C under vacuum. The molecular motion is very much affected by crosslinking, that is, the logarithmic decrement of β relaxation (19 and 30°C) quickly disappears, the γ relaxation (−97°C) shifts to a higher temperature and α relaxation (130°C) shifts to a lower temperature, as the crosslinking density increases. The γ relaxation is assigned to be a glass transition temperature and the α relaxation to be the transition in molecular motion of inter-polymer chains among either crystallites or crosslinking sites.

Temperature effect on radiation induced reactions in ethylene and tetrafluoroethylene copolymer (ETFE)

Abstract Ethylene and tetrafluoroethylene copolymer (ETFE) was irradiated by γ-rays or electron beam (EB) under oxygen-free atmosphere at various temperatures ranging from 77 to 573 K. Mechanical and thermal properties, and absorption spectra of the irradiated ETFEs were measured. The mechanical properties of the film have been observed to change by irradiation. The modulus and yield strength increase with increasing dose, and these phenomena are clearly distinguished above the melting temperature of ETFE (533 K). Heat of crystallization changes drastically as a function of irradiation dose around the melting temperature, compared with other temperatures. The absorption band around 250 nm of irradiated ETFE shifts to a longer wavelength region with increase of irradiation temperature. Therefore, it was concluded from those experimental results mentioned above that crosslinking takes place and conjugated double bonds formation proceeds in a wide range of irradiation temperatures. Those reactions are enhanced by increasing temperature. The homogeneous crosslinking takes place in the molten state, while the heterogeneous crosslinking does in the crystalline solid state.

Modification of natural rubber by different grafting techniques

Abstract An attempt has been made to graft a hydrophylic monomer of N,N.dimethyl acrylamide (DMAA) onto natural rubber (NR) tube by simultaneous, per-oxidation and preirradiation grafting techniques. It was found that the grafting by simultaneous grafting technique results a maximum 29 wt% degree of grafting and by peroxidation and preirradiation techniques, results the maximum 42 wt% and 13 wt% degree of grafting, respectively. It was concluded that the peroxidation technique is the most appropriate to obtain a high degree of grafting in radiation copolymerization of DMAA onto NR.